At present, nanoparticles such as oxide nanoparticles, nitride nanoparticles and carbide nanoparticles are used in the production of semiconductor substrates, printed circuit boards, electrical insulation materials for various electrical insulation parts and the like, cutting tools, dies, bearings and other high-hardness and high-precision machining tool materials, functional materials for humidity sensors and the like, and sintered bodies for use as precision sinter molding materials, and in the production of thermal sprayed parts such as engine valves made of materials that are required to be wear-resistant at a high temperature, as well as in the fields of electrode or electrolyte materials and various catalysts for fuel cells. The use of those nanoparticles improves bonding strength between different ceramics or different metals and denseness as well as functionalities of sintered bodies, thermal sprayed parts and the like.
The above-described nanoparticles are produced by a chemical approach in which various gases or the like are chemically reacted at high temperature or a physical approach in which substances are irradiated with an electron beam, laser beam or the like to be decomposed and vaporized, thereby generating nanoparticles. The nanoparticles produced by the above approaches have a particle size distribution and contain coarse powder and fine powder mixed together. The nanoparticles used in the applications described above preferably has a smaller proportion of coarse powder, since excellent properties can be achieved. Accordingly, a powder classifying apparatus that uses a whirling stream to impart whirling motion to powder such that the powder is centrifuged into coarse powder and fine powder has been utilized (see Patent Literature 1 , for example).
Patent Literature 1 describes a powder classifying apparatus in which powder having a particle size distribution is carried by an air stream and supplied. The powder classifying apparatus of Patent Literature 1 includes a hollow cavity in a disc-like shape (disc-like cavity portion) in which the supplied powder having a particle size distribution is classified, a powder supply port for supplying the powder having a particle size distribution to the disc-like cavity portion, a plurality of guide vanes arranged so as to each extend from an outer circumference of the disc-like cavity portion in an inward direction at a given angle, a discharge unit for an air stream including fine powder discharged from the disc-like cavity portion, a collection unit for coarse powder discharged from the disc-like cavity portion, and a plurality of air nozzles arranged on an outer circumferential wall of the disc-like cavity portion along a tangential direction of the outer circumferential wall below the guide vanes and blowing compressed air toward the collection unit for coarse powder inside the disc-like cavity portion to bring fine powder present near the collection unit for coarse powder back to the disc-like cavity portion.